Melting furnace and method

ABSTRACT

An aluminum can melting furnace in which a slip stream of molten material is diverted from a main molten bath in the furnace, a vortex is formed in the diverted slip stream, and thin walled aluminum can feedstock is introduced into the vortex. The feedstock is immediately drawn under the surface of the vortex in the slip stream so the feedstock melts before it has a chance to oxidize. Hydrocarbons which are introduced with the feedstock are flash vaporized in the vortex. The hydrocarbon vapors are captured and conducted to the furnace burner where they are burned to increase the efficiency of the operation and reduce the pollution generated by the process.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to apparatus and method for meltingfurnaces, and, in particular, to method and apparatus for meltingfurnaces which are particularly adapted to the efficient melting offinely divided materials in the presence of oxygen without excessiveoxidation.

2. Description of the Prior Art

Previous furnaces for melting finely divided materials in air, such as,for example, thin walled aluminum cans, often permitted excessiveoxidation of the feedstock. This significantly reduced the efficiency ofthe operation. Previous expedients generally introduced the feedstockonto the surface of a molten mass and allowed the finely divided feedstock to melt down into the mass. Thin walled aluminum can feedstock,for example, oxidizes rapidly, and sometimes even ignites, as soon as itreaches its melting point. Because of the physical form of thin walledcans they are particularly difficult to melt efficiently.

Many times, for example, thin walled can feedstock, as delivered to amelting furnace, contains hydrocarbon materials in the form of paint orlacquer on the can, or residues of product within the can. Previously,disposing of these hydrocarbons safely, efficiently and withoutpolluting was difficult.

Melting furnaces are generally operated to turn finely divided metallicfeedstock into ingots. Previously it had been generally impossible tooperate such melting furnaces efficiently in ambient air withoutreleasing significant pollutants.

These and other difficulties of the prior art have been overcomeaccording to the present invention.

BRIEF SUMMARY OF THE INVENTION

The melting furnaces and methods according to the present inventiongenerally comprise a melting furnace, for example, in which theatmosphere is ambient air, and includes a chamber for holding a moltenmass, a heat source, and some means for forcibly immersing feedstockbelow the surface of the molten mass as soon as it is introduced intothe furnace, and in any event, before the feedstock is heated to thetemperature where rapid oxidation occurs. In general the melting furnaceis provided with particulate and other pollution control devices for thegas phase exhaust which is discharged from the furnace.

According to one preferred embodiment of the present invention, a metalmelting furnace which is open to the ambient air includes a furnacechamber for holding a molten mass, a burner for supplying heat to themolten mass in the furnace chamber, an external circulation loop forconducting a slip stream of the molten mass away from and returning itto the furnace chamber, a vortex chamber in the external circulationloop, a vortex generator associated with the vortex chamber, a feedstocksupply port associated with the vortex chamber, and a hydrocarbon vaporcollection system associated with the vortex chamber for collectinghydrocarbon vapor and delivering it to the burner.

According to one form of the invention, the atmosphere within thefurnace chamber is drawn at least in part from the ambient atmosphere sothat it contains some oxygen. At the operating temperatures involved inmelting metals, oxidation occurs very rapidly.

In general the intended feedstock is finely divided so that it presentsmany times the surface area of the top surface of the bath of moltenmass. Thin walled aluminum cans, crushed, uncrushed, chopped or groundare finely divided as are powders, granules and chopped materials. Ingeneral, the feedstock is metallic in nature and the desired output fromthe furnace is the metal in ingot form. The feedstock, because of itsfinely divided physical form, is generally more susceptible to oxidationthan the bath, even when the exposed surface of the bath is unprotected.Thus, the feedstock must generally be protected from oxidation more thanthe bath. The top surface of the bath may, if desired, be protected fromoxidation, for example, by forming a molten blanket of some othermaterial, such as, for example, a molten salt, on its surface. Thefinely divided feedstock is protected from oxidation by immersing it inthe molten mass as soon as it is introduced into the furnace and beforeit reaches its melting point.

The external circulation loop, for example, carries a stream of themolten mass away from the furnace chamber, through a vortex chamber, andback to the furnace chamber. Feedstock is conveniently introduced to thefurnace in this external circulation loop. Various reactants for variouspurposes may also be introduced into the diverted stream of molten massas it flows through the external circulation loop. In general the streamis moved through the external loop under the urging of some pumpingaction. Because some cooling usually takes place in the external loop itis generally preferred, although not essential, that the stream bereturned to the furnace chamber at some location which is physicallyremoved from the location where the stream is withdrawn from the furnacechamber. For purposes of convenience and simplicity of design it isgenerally preferred that the external loop conduct the stream along apath which is generally level and at approximately the same horizontallevel as the bath in the furnace chamber. The stream may, however, ifdesired, be pumped to different levels.

The vortex generator creates a vortex in the vortex chamber. The vortexis formed in the stream of molten mass as it flows through the externalcirculating loop. The finely divided feedstock is introduced through afeedstock supply port to a location where the feedstock is immediatelydrawn into the vortex. The vortex draws the feedstock under the surfaceof the molten stream where it is quickly heated to its melting point outof contact with air. The feedstock becomes part of the stream of moltenmass and flows back to the furnace chamber along the externalcirculation loop. The diverted stream of molten mass may be composed ofmolten feedstock alone or it may include or be composed entirely of bathblanket material, if a blanket is present, and molten feedstock.Oxidation of the feedstock is thus minimized and a maximum amount of thefeedstock is recovered in the desired metallic form.

The finely divided feedstock often carries with it some hydrocarbonmaterials. The temperature of the molten mass in the vortex issufficiently high to cause the hydrocarbon materials to vaporizeimmediately and escape from the molten mass. Preferably, the vaporizedhydrocarbons are collected in a headspace over the vortex, and thatheadspace is vented to the fuel supply for the burner which heats themolten mass in the furnace chamber. If desired, the vapors may becollected downstream from the vortex chamber. Thus, the hydrocarbonmaterials in the feedstock serve to reduce the amount of purchased fuelwhich is required to heat the furnace chamber. Where the feedstockcontains hydrocarbons which will be vaporized, it is generally preferredthat the collection site, for example, the headspace portion of thevortex chamber, be sealed from the flame of the burner so as to preventany unevenness in the rate of the generation of the vapor from creatingan explosive mixture which might be ignited by the open flame of theburner.

The vortex generator may conveniently take the form of a pump. Accordingto a preferred embodiment the vortex generator is in the form of apaddle which rotates at a rate which is sufficient to form the vortex inthe molten mass and drawn the feedstock down immediately upon itsintroduction. The vortex generator may also serve to pump the stream ofmolten mass through the external circulating loop.

The feedstock may contain elements which are not desired in the metallicingots which are the intended end product of the process. If, forexample, the metallic ingots are intended to be aluminum and thefeedstock contains, for example, some copper, various materials may beadded to react with the copper to place it in a separable form. Removalof such elements may be accomplished, for example, by adding substanceswhich react with the undesired elements to form compounds which may bephysically separated from the molten bath. Preferably, the compounds areimmiscible with and of a different density from the molten feedstock sothat they form a removable layer above or below the layer of moltenfeedstock in the furnace chamber. The reactants are conveniently addedto the stream of molten mass in a reaction chamber along the externalcirculation loop.

Melting furnaces according to the present invention are particularlysuited for use where the atmosphere in the furnace chamber includesoxygen, the heat source is a burner, and the feedstock is a metal. Suchfurnaces are, however, also suitable for use in other reactive ambientatmospheres and with nonmetallic feedstock materials. Other heatsources, such as, for example, induction heating and the like, may beemployed, if desired. Such furnaces may also be useful where thefeedstock is subject to oxidation for reasons in addition to or apartfrom its physical form.

The feedstock may be delivered to the furnace by means, for example, ofa gravity feed chute, a screw feeder, or the like. The feedstock supplyport is generally positioned so as to discharge feedstock directly intothe headspace over the vortex so that the entering feedstock isimmediately drawn under the surface of the molten mass. The feedstocksupply port preferably is at least partially sealed so as to prevent theescape of the gas phase from the headspace region of the vortex chamberinto the atmosphere.

Generally, the furnace is placed in operation by heating a mass ofmaterial which may be feedstock or bath blanket material, or both, untilit is well above its melting point. The vortex generator is activated soas to form the vortex, and feedstock is introduced into the vortex. Theoperation is preferably conducted as a continuous process with feedstockbeing introduced continuously, although it may be operated as a batchprocess, if desired.

According to an alternative and generally less preferred arrangement thefeedstock supply port directs feedstock generally into a submersiblebasket which is positioned generally above the molten mass. As soon asthe submersible basket is loaded with finely divided feedstock it issubmerged within the molten mass, carrying the feedstock with it. Afterthe feedstock is melted the submersible basket is withdrawn from themolten mass and recharged with feedstock. The submersible basket ispreferably positioned in an external circulation loop but it may bepositioned within the furnace chamber. In general a vortex need not beformed when a submersible basket is employed. The batch nature of thesupply of the feedstock risks oxidizing the feed stock before it isimmersed in the bath. In general the finely divided feedstock is subjectto rapid oxidation, that is it may ignite or burn in less than 30seconds after being introduced into the furnace. If the basket is notsubmerged within less than 30 seconds some of the feedstock willprobably be lost due to oxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring particularly to the drawings for the purposes of illustrationand not limitation:

FIG. 1 is a cross-sectional simplified schematic view of a preferredembodiment of the invention.

FIG. 2 is a cross-section view taken along line 2--2 in FIG. 1.

FIG. 3 is a cross-sectional view similar to FIG. 2, of a differentfurnace.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring particularly to the drawings, in the preferred embodimentwhich is referred to for purposes of illustration, in FIGS. 1 and 2,there is schematically illustrated generally at 10, a melting furnace.Melting furnace 10 includes, for example, burner 12, and a melting orfurnace chamber 14. A vortex chamber 16 and a mixing chamber 18 arepositioned, for example, along an external circulation loop 20. Loop 20extends between exit port 22 and entrance port 24. Vortex chamber 16includes a headspace region 26. A feedstock supply port 28 is preferablypositioned so as to deliver feedstock 30 to the vortex chamber 16. Avortex generator, for example, paddle 32, rotatably driven by shaft 34,is positioned in vortex chamber 16. The heat generated by the burner 12causes material in furnace chamber 14 to melt so as to form a moltenmass in the form of a bath 38. The molten mass fills externalcirculation loop 20, including vortex chamber 16 and mixing chamber 18.In vortex chamber 16 the rotation of paddle 32 causes the formation of avortex 36 in the stream of molten material which is flowing through theexternal circulation loop 20. The headspace region 26 in vortex chamber16 is, for example, vented to burner 12 by means of a vapor conduit 40,whereby any vaporized hydrocarbons which appear in headspace region 26are burned as auxiliary fuel in burner 12.

Melting furnace 10 is conveniently constructed of conventional furnaceconstruction materials. Paddle 32 may conveniently be constructed, forexample, of ceramic materials which are substantially unaffected byeither the operating temperatures within the furnace or the molten masswithin the furnace.

This invention finds particular utility where the desired metallicingots are aluminum and the feedstock is thin walled cans. In thispreferred operation the burner 12 operates at a temperature ofapproximately 1700 degrees Fahrenheit and the bath 38 is at atemperature of approximately 1375 degrees Fahrenheit. The bath 38 mustbe at a temperature which is sufficiently above the melting point of thealuminum to prevent the slip or side stream in the external circulationloop 20 from solidifying while it is separated from the main bath. Whena sufficient quantity of the feedstock 30 has been melted the furnace istapped to release a stream of the molten feedstock into an ingot formingreceptacle (not shown).

The paddle 32 is rotated at a rate which is sufficient to establish astrong vortex 36. When the bath 38 is primarily molten aluminum and thebath is approximately 10 inches deep the paddle should be rotated at arate of from approximately 400 to 1500 revolutions per minute. Arotational rate of 400 revolutions per minute is generally sufficient toestablish the vortex 36. If substantial quantities of feedstock 30 areintroduced at a rapid rate the rate of rotation of the paddle 32 mayhave to be increased to as much as 1500 revolutions per minute so as tosubmerge the feedstock quickly enough to prevent excessive oxidation.The action of the paddle 32 also serves to pump the molten mass alongthe external circulation loop 20. This pumping action needs to be quitestrong where the slip stream is being cooled quickly by the rapidintroduction of cold feedstock into the vortex. If the liquid phase slipstream were to be cooled to the point of phase transition to the solidphase the results would be disastrous to the operation of the furnace.The efficiency of the operation is improved and a pollution controlproblem is eliminated by using the hydrocarbon vapors which aregenerated during the melting of cans to help heat the bath 38 in chamber14.

Any lacquer, paint or other organic residues which are introduced withthe feedstock are generally volutilized very rapidly so that they escapefrom the molten mass into the headspace region 26. Submersing thefeedstock immediately tends to improve the yield of hydrocarbon vaporsand reduce the production of elemental carbon. This improves theefficiency of the operation because the hydrocarbon vapors are directedto the burner 12 where they are burned off as auxiliary fuel. This alsoavoids the necessity of collecting these hydrocarbons in some othervapor or solid phase to prevent them from being vented to theenvironment. The gas phase exhaust from the furnace is treated byconventional pollution control equipment and procedures.

Referring particularly to FIG. 3, a melting furnace is illustratedschematically at 42. Melting furnace 42 includes a burner 44, a furnacechamber 52, a feedstock supply port 46, and a perforated submersiblebasket 48 mounted for reciprocal motion with reciprocating shaft 50.Heat generated by burner 44 melts the material which is in furnacechamber 52 to form a molten mass indicated generally at 54. In theembodiment chosen for illustration in this embodiment the molten mass 54is composed of a blanket layer 56 and a layer 58 of molten metal. Theblanket layer, which may conveniently be molten salt, serves to protectthe surface of the molten metal layer 58 from oxidation. Finely dividedfeedstock 60 is introduced through supply port 46 into basket 48 whichis then submerged below the surface of molten mass 54 by the action ofshaft 50. When the feedstock in basket 48 has been rendered molten thebasket is raised by shaft 50 to a position to receive more feedstock. Ifvolatile hydrocarbons are released from the feedstock submersion shouldtake place in a separate chamber where the vapors are no exposed to theopen flame of the burner 44.

Molten bath blankets may be used with any form of the present inventionalthough they are generally not needed where aluminum is the desiredproduct.

What has been described are preferred embodiments in whichmodifications, changes, substitutions and reversals may be made withoutdeparting from the spirit and scope of the accompanying claims.

What is claimed is:
 1. A furnace for melting finely divided oxidizablefeedstock comprising:a chamber adapted to hold a molten mass which isgenerally exposed to air, a heat source positioned to supply heat tosaid molten mass, a vortex region adapted to contain a vortex formed ofsaid molten mass, a vortex generator associated with said vortex regionand adapted to generate said vortex including a vortex surface, and afeedstock supply port positioned to discharge finely divided oxidizablefeedstock onto said vortex surface.
 2. A furnace for melting finelydivided oxidizable feedstock comprising:a chamber adapted to hold amolten mass, a heat source comprising a burner positioned to supply heatto said molten mass, a vortex region generally isolated from said burnerand adapted to contain a vortex formed of said molten mass, a vaporconduit positioned to conduct vapors from generally said vortex regionto said heat source, and a feedstock supply port positioned to dischargefinely divided oxidizable feedstock into said vortex region.
 3. Afurnace for melting finely divided oxidizable feedstock comprising:afurnace chamber adapted to hold a molten mass, a heat source positionedto supply heat to said molten mass, an external circulation looppositioned to receive said molten mass and conduct it away from and backto said furnace chamber, a vortex chamber positioned along said externalcirculation loop and adapted to contain a vortex formed of said moltenmass, and a vortex generator associated with said vortex chamberpositioned to generate a vortex having a vortex surface in the moltenmass in said vortex chamber, and a feedstock supply port positioned todischarge finely divided oxidizable feedstock onto said vortex surface.4. A method of melting finely divided oxidizable feedstockcomprising:applying heat to a material in a first region to form amolten mass, said heat being generated at least in part by burning ahydrocarbon containing fuel in a burner, forming a vortex in said moltenmass in a second region, said second region including a generallyenclosed headspace above said vortex, said enclosed headspace beingisolated from said burner, said vortex including a vortex surface,adding finely divided oxidizable feedstock including hydrocarbonmaterials to said vortex surface, and allowing said feedstock to bedrawn under the surface of the molten mass by the vortex before it isheated to a temperature at which it would be rapidly oxidized, wherebyoxidation of said finely divided feedstock is minimized, allowing saidhydrocarbon materials to vaporize and collect in said enclosedheadspace, and conducting the resultant vaporized hydrocarbon materialsaway from said enclosed headspace.
 5. A method of melting finely dividedoxidizable feedstock comprising:applying heat to a material in thepresence of air to form a bath comprising a molten mass of saidmaterial, diverting a stream of said bath to a vortex forming location,forming a vortex in said stream at said vortex forming location, saidvortex having a vortex surface, adding finely divided oxidizablefeedstock onto said vortex surface, and allowing said feedstock to bedrawn under the surface of the stream by the vortex before the finelydivided oxidizable feedstock is heated to a temperature at which itwould be rapidly oxidized, whereby oxidation of said finely dividedfeedstock is minimized, and returning said stream to said bath.
 6. Amethod of claim 5 wherein said feedstock includes hydrocarbon material,and said method of melting includes conducting vapor phase hydrocarbonmaterial away from said stream of molten mass.
 7. A method of claim 6wherein said oxidizable feedstock is finely divided.